Origin of long-lived oscillations in 2D-spectra of a Quantum Vibronic Model: Electronic vs Vibrational coherence

TL;DR

This paper investigates the origins of long-lived oscillations in 2D spectra of photosynthetic complexes, showing that both electronic and vibrational coherences, influenced by excitonic interactions, contribute significantly to observed signals.

Contribution

It provides a detailed analysis of electron-phonon dynamics in a dimer model, highlighting the comparable roles of electronic and vibrational coherences in realistic biological systems.

Findings

Pure electronic and vibrational coherences are of similar magnitude.

Vibrational coherence depends on excitonic interactions.

Results align with experimental 2D spectroscopy observations.

Abstract

We demonstrate that the coupling of excitonic and vibrational motion in biological complexes can provide mechanisms to explain the long-lived oscillations that have been obtained in non linear spectroscopic signals of different photosynthetic pigment protein complexes and we discuss the contributions of excitonic versus purely vibrational components to these oscillatory features. Considering a dimer model coupled to a structured spectral density we exemplify the fundamental aspects of the electron-phonon dynamics, and by analyzing separately the different contributions to the non linear signal, we show that for realistic parameter regimes purely electronic coherence is of the same order as purely vibrational coherence in the electronic ground state. Moreover, we demonstrate how the latter relies upon the excitonic interaction to manifest. These results link recently proposed microscopic, non-equilibrium mechanisms to support long lived coherence at ambient temperatures with actual experimental observations of oscillatory behaviour using 2D photon echo techniques to corroborate the fundamental importance of the interplay of electronic and vibrational degrees of freedom in the dynamics of light harvesting aggregates.